Zac Efron's friends are concerned

The '17 Again' star - who has reportedly been to rehab twice this year to help with his supposed cocaine and alcohol addiction - has left those close to him concerned in case he suffers a relapse similar to the one which killed 'Glee' actor Cory Monteith in July.

A source told RadarOnline.com: "Friends are genuinely worried about him, because if Zac relapses like Cory Monteith did he'll likely go back to using the same amount of drugs he was doing before rehab. And it will kill him."

Now Zac is said to be clean and sober, friends are hoping he won't slip back into his old habits, which had seen him "on a downward spiral".

The insider added: "Zac had been on a downward spiral since 2011, he was just good at hiding it and not getting caught.

"He was partying because he was unhappy and basically just didn't give a f**k."

The 25-year-old actor looks to be in a better place after he was spotted on at the premiere for his new movie 'Parkland' at the Toronto International Film Festival earlier this week.